1. Field of the Invention
This invention relates to semiconductor fabrication and more particularly to an improved process of planarizing a field dielectric upper surface commensurate with adjacent active regions. The improved planarization process involves etching trenches about the field region to form field mesas, and then oxidizing the field mesas and trenches to produce a field dielectric which extends in planar fashion entirely across the field region.
2. Description of the Relevant Art
The fabrication of an integrated circuit involves placing numerous devices in a single semiconductor substrate. Select devices are interconnected by a conductor which extends over a dielectric which separates or xe2x80x9cisolatesxe2x80x9d those devices. Implementing an electrical path across a monolithic integrated circuit thereby involves selectively connecting isolated devices. When fabricating integrated circuits it must therefore be possible to isolate devices built into the substrate from one another. From this perspective, isolation technology is one of the critical aspects of fabricating a functional integrated circuit.
A popular isolation technology used for an MOS integrated circuit involves the process of locally oxidizing silicon. Local oxidation of silicon, or LOCOS process involves oxidizing field regions between devices. The oxide grown in field regions are termed field oxide, wherein field oxide is grown during the initial stages of integrated circuit fabrication, before source and drain implants are placed in device areas or active areas. By growing a thick field oxide in isolation (or field) regions pre-implanted with a channel-stop dopant, LOCOS processing serves to prevent the establishment of parasitic channels in the field regions.
While LOCOS has remained a popular isolation technology, there are several problems inherent with LOCOS. First, a growing field oxide extends entirely across the field region and laterally as a bird""s-beak structure. In many instances, the bird""s-beak structure can unacceptably encroach into the device active area. Second, the pre-implanted channel-stop dopant of ten times redistributes during the high temperatures associated with field oxide growth. Redistribution of channel-stop dopant primarily affects the active area periphery causing problems known as narrow-width effects. Third, the thickness of field oxide causes large elevational disparities across the semiconductor topography between field and active regions. Topological disparities cause planarity problems which become severe as circuit critical dimensions shrink. Lastly, thermal oxide growth is significantly thinner in small field (i.e., field areas of small lateral dimension) regions relative to large field regions. In small field regions, a phenomenon known as field-oxide-thinning effect therefore occurs. Field-oxide-thinning produces problems with respect to field threshold voltages, interconnect-to-substrate capacitance, and field-edge leakage in small field regions between closely spaced active areas.
Many of the problems associated with LOCOS technology are alleviated by an isolation technique known as the xe2x80x9cshallow trench processxe2x80x9d. Despite advances made to decrease bird""s-beak, channel-stop encroachment and non-planarity, it appears that LOCOS technology is still inadequate for deep submicron MOS technologies. The shallow trench process is better suited for isolating densely spaced active devices having field regions less than one micron in lateral dimension.
The trench process involves the steps of etching a silicon substrate surface to a relatively shallow depth, e.g., between 0.2 to 0.5 microns, and then refilling the shallow trench with a deposited dielectric. Some trench processes include an interim step of growing oxide on trench walls prior to the trench being filled with a deposited dielectric. After the trench is filled, it is then planarized to complete the isolation structure.
The trench process eliminates bird""s-beak and channel-stop dopant redistribution problems. In addition, the isolation structure is fully recessed, offering at least a potential for a planar surface. Still further, field-oxide thinning in narrow isolation spaces does not occur and the threshold voltage is constant as a function of channel width.
While the trench isolation process has many advantages over LOCOS, it cannot in all instances achieve complete global planarization across the entire semiconductor topography. The upper surface of fill dielectric in large isolation areas are generally at lower elevational levels than the upper surface fill dielectric in small isolation areas. The fill dielectric readily deposits in small area trenches such that the elevation of the fill dielectric in a small area is greater than the elevation within a large area trench. Accordingly, subsequent processing is needed to bring the large area fill topography to the same elevational level as small area fill topography.
Most researchers have focused upon fairly complex processes for planarizing large and small area fill dielectrics. Those processes generally involves rework of the fill dielectric. A popular rework technique involves depositing a sacrificial layer across the fill dielectric topography, and then removing the sacrificial layer at the same etch rate as the underlying dielectric. Generally, the sacrificial layer is deposited as a low-viscosity liquid. Baking the liquid, or exposing it to ultraviolet light, causes the liquid to convert to solid form in a sol-gel reaction. Popular sacrificial materials include photoresist, polyimide or spin-on glass (SOG). The sacrificial layers generally etch back in a plasma until the topmost regions of the fill dielectric are exposed. The etch chemistry is then modified so that the sacrificial layer material and the underlying fill dielectric are etched at approximately the same rate. The etch is continued until all of the sacrificial layer has been etched away, leaving a somewhat planar dielectric upper surface.
The sacrificial etchback technique is generally valid only for the planarization of topographies in which features are less than 10 xcexcm (microns) apart. For large regions between trenches, the step height will not be reduced, since the photoresist thickness on top of such features will be the same as the thickness over the adjacent trench.
In an effort to overcome the shortcomings of the etchback process, a planarization block mask may be used. In this procedure, a liquid material is applied and developed as a planarization film followed by a block mask used to expose and develop this film. The block mask protects topography in wide, low regions from a subsequent etch plasma. The upper surface of high regions can then be removed to an elevational level commensurate with the protected low regions. The planarization block mask involves an additional lithography step and a mask which is produced by selectively reversing the mask used in producing the underlying topography. Mask reversal may involve errors due to changes in the resist thickness caused by the underlying pattern, misalignment, etc.
In an effort to eliminate the complex deposition, patterning and etch processes involved with sacrificial deposition, etch back, and the block masking, many manufacturers have directed their attention to chemical-mechanical polishing (CMP). Application of a chemical slurry and an abrasive polishing pad across the entire semiconductor topography, allows planarization of that topography commensurate with the planarity of the pad surface. Unfortunately, however, when force is applied to a pad, the pad will oftentimes conform to the unevenness of that topography. Thus, while high elevation areas, or peaks, receive substantial polishing, low elevational areas (or valleys) are also abraded.
A better understanding of the problems inherent with CMP are illustrates in reference to FIGS. 1 and 2. FIG. 1 depicts a partial cross-section of a semiconductor topography 10. Topography 10 includes a substrate 12 which has been fashioned with a small area trench 14 and a large area trench 16 according to the trench isolation process. Trenches 14 and 16 define field areas between active regions, wherein active regions are defined as silicon mesas 18 which extend from an elevation equivalent to the trench base. Deposited upon and between mesas 18 is a fill dielectric 20.
FIG. 2 illustrates a processing step subsequent to that shown in FIG. 1. Specifically, the upper surface of fill dielectric material 20 receives CMP. The polishing pad inherently flexes or conforms under pressure to the upper surface of dielectric 20, causing the polishing pad to attack and remove dielectric 20 upper surface in large area trench 16, albeit to a lesser degree than the removal of dielectric 20 in small area trench 14. If dielectric in large area trench 16 is not sufficiently thick to withstand the attack, the dielectric upper surface will be removed below the desired planar elevation. Thus, a slight recess of dielectric 20 upper surface occurs at the conclusion of CMP. That recess is shown as reference numeral 22. Isolated mesas may also show erosion of the silicon surface due to CMP.
A need therefore exists in producing a process which can utilize the advantages of the shallow trench isolation technique. The desired process must, however, not bear the disadvantages of a non-planar shallow trench. More specifically, an improved process must be derived which does not produce the problems inherent in using a fill dielectric followed by planarization rework. Thus, the desired process must be one which avoids having to use, for example, a sacrificial etchback, block mask and/or CMP.
Avoiding conventional planarization processes thereby avoids the problems of not being able to achieve complete global planarization. Global planarization is defined as the planarization of the field dielectric between active areas, regardless of the relative size differences and densities between field region areas. More specifically, a process must be derived which can planarize field dielectric in large isolation regions as well as small isolation regions. Large area field regions are those defined as having a width or length greater than 16.0 microns. Field regions are defined as those regions which do not contain active devices. All active devices (e.g., transistors, etc.) are formed in active regions which reside between field regions. A need therefore exists for forming a field dielectric upper surface that is substantially planar and equivalent to all other field dielectric and active area upper surfaces which extend across the entire wafer topography. The desired process thereby achieves global planarization with minimal planarization steps.
The problems outlined above are in large part solved by an improved shallow trench process of the present invention. The improved process hereof serves to form silicon mesas in active areas by forming a trench of defined width immediately adjacent the periphery of each active area. The trench is etched into the silicon substrate within the field region, leaving a silicon mesa in each active region, but also leaving a silicon mesa in a field region. A silicon mesa within an active region is herein defined as an xe2x80x9cactive mesaxe2x80x9d and a silicon mesa within a field region is herein defined as a xe2x80x9cfield mesaxe2x80x9d. Thus, instead of forming a trench entirely across the field region as in conventional processes, the present process forms a narrow trench only at the periphery of the field region.
The field mesa can be of varying size, depending upon the field region dimension. Accordingly, in large field regions, the field mesa is also large. In small field regions, the field mesa can be small, or non-existent. In all instances, the trench formed within the field region, at the juncture between active and field regions is of a pre-defined width. The trench width is fixed, preferably between 0.1 to 1.0 microns. The fixed, defined width of the trench is necessary to ensure a trench fill during subsequent field dielectric formation.
The field mesa includes an upper surface which is substantially planar with respect to the laterally spaced active mesas. The field mesa upper surface, being silicon, allows growth of planarizing (and/or fill) dielectric material thereon. The field dielectric is placed upon the field mesa and in trenches which surround each field mesa in a step subsequent to the step used in forming the field mesa. The field dielectric serves to prevent inversion of the field area by a powered, overlying interconnect. The field dielectric extends between active area silicon mesas, entirely across the dielectric-filled trenches and dielectric-covered field mesas.
Preferably, the field dielectric is a thermally grown oxide. The oxide forms as a result of removing an oxide barrier layer from the field mesas but not from the silicon mesas in active areas. When the field oxide grows, a curvature occurs at the field oxide upper surface immediately adjacent the active area. Presence of the curvature serves to minimize the elevational step of an interconnect extending from the active region to the field region. Preferably, the field oxide is grown to a thickness of 0.2 to 0.4 microns. As the field oxide grows, it partially consumes the field mesa upper and sidewall surfaces leaving a larger critical dimension between the silicon field areas and overlying interconnect extending into active regions.
Broadly speaking, the present invention contemplates a method for forming a field dielectric. The method includes the steps of providing a silicon substrate having a field region bounded by at least one active region. A trench is then formed of defined width within the field region immediately adjacent the active region to form a field mesa. The field mesa is completely bounded by the trench. A field dielectric is then grown upon the field mesa and within the trench. The resulting field dielectric thereby includes an upper surface which is substantially planar across the field region commensurate with, and slightly above, the upper surface of the active region. Advantageously, the field dielectric is planar across the field mesa, regardless of the field mesa lateral area. As such, the present method achieves a field dielectric upper surface which is at substantially the same elevation entirely across the field area, regardless of field area size.
The present invention further contemplates a field region comprising a field mesa extending from a silicon substrate a lateral spaced distance from an active area. The field region further includes a field dielectric which covers the field mesa and fills the lateral spaced distance between the field mesa and the active area.
Still further, the present invention contemplates a field region comprising a trench extending into a silicon substrate about the periphery of the field region to form a field mesa. A field dielectric is configured across the field mesa and the trench. The field dielectric thereby fills the trench and leaves a substantially planar upper surface which extends entirely across the field region.